Two Paleozoic orogenic cycles preserved in the Central Alpine basement (Central Alps, Switzerland)

We investigated the Paleozoic evolution of basement units in the northern and southern Aar Massif to provide new insights into its Ordovician and Carboniferous (Variscan) tectonic and metamorphic evolution. The northern and the southern basement units contain former large volumes of melts (more as 80%), which locally develop a granitoid character, and therefore are partly interpreted as granites and partly as diatexites. The dominant quartz-feldspathic composition of the units in combination with the Alpine overprint does not allow a detailed quantitative determination of the physical conditions of metamorphism.

A detailed laser ablation ICPMS geochronological study provides evidence for two episodes of high-temperature metamorphism and anatexis: Both localities are dominated by Ordovician-age zircon, occurring as (i) oscillatory-zoned (OZ) thick rims overgrowing ubiquitous Neoproterozoic (~600-800 Ma) cores; (ii) as entire OZ and sector-zoned (SZ) grains of magmatic appearance, or as (iii) OZ and SZ cores overgrown by thin OZ rims of younger age. Mean weighted ²⁰⁶Pb/²³⁸U ages from OZ zones cluster between 447 and 455 Ma (± ~2-3 Ma internal error). Zircon from the “Erstfeld gneiss” in the northern Aar massif almost lacks any record of a Variscan-age metamorphic overprint. A pervasive ductile fabric in the rock is interpreted as the result of a solid-state deformation during Variscan medium-grade metamorphism. Beside the dominant record of Ordovician magmatism, the samples from the southern part of the Aar massif contain zircon with U-rich, often OZ rims that crystallized during a Variscan high-temperature overprint in the presence of melt. The age of the Variscan overprint is poorly defined due to superimposed lead loss. These rocks have been variably mapped as diatexite or granite (e.g., the Strem granite in the eastern part of this zone). They contain subhedral allanite, as well as allanite overgrowing britholite. This indicates the presence of a melt fraction in the stability field of allanite, probably during the Variscan cycle. The studied units are the country rocks of pulses of Variscan magmatism at 335, 310 and 300 Ma [1]. In-situ U-Pb ages of U-rich zircon rims from the country rocks range from 320 to 310 Ma, roughly coinciding with the emplacement time of the intrusive rocks. We therefore conclude that the units investigated from the northern and southern parts of the Aar massif underwent extensive anatexis in Ordovician time, but were at different crustal levels during the Variscan orogeny.

We also carried out in-situ analysis of the zircon Hf isotopic composition in the same growth zones that were dated by U-Pb. The variation in eHf in inherited Neoproterozoic cores, Ordovician growth zones and Variscan rims is quite similar, providing evidence for a process of crustal recycling without the participation of a mantle component, neither during the Ordovician nor during the Variscan cycle. This raises questions about the nature of the thermal energy required to cause such large-scale crustal melting. Further studies will need to focus on the question of the source of heat (and water) for such melting events.

[1] Ruiz et al. (2022) Swiss Journal of Geosciences, 115, 20